Geothermal gas geochemistry in Southeast Tibetan Plateau margin influenced by magma chambers and fractures

Abstract

Hydrothermal gases play a pivotal role in elucidating the cycling of deep materials and subsurface processes. Here, we analyzed the chemical and isotopic compositions of 40 gas samples and compiled geochemical data on >280 samples discharged from the Tengchong hydrothermal field, located on the Southeast Tibetan Plateau margin. Helium isotopes span from 0.13 R_a (the ³He/⁴He ratio of air) to 5.91 R_a, indicating a mixing between mantle-derived helium and crustal components. Spatial distributions of helium isotopes are closely associated with the locations of magma chambers beneath the Tengchong field. N₂/Ar ratios and nitrogen isotopic compositions suggest that N₂ and Ar were primarily contributed by atmospheric sources and groundwater, with minor N₂ contributions from mantle-derived materials. CO₂ was mostly originated from the thermal decomposition of limestone and magma degassing, with a few samples affected by secondary processes like carbonate precipitation. The chemical and carbon‑hydrogen isotopic compositions of alkanes indicate that methane was mainly sourced from thermogenic processes. The central magma chamber and its adjacent faults are characterized by the most intensive magmatic activity and the closest connection to deep materials. A gas migration and diffusion model using a crustal ³He endmember of 6 × 10⁻⁹–9 × 10⁻⁸ (volume ratio of ³He to total gases) well explains the decreasing ³He concentrations with the distance from magma chambers. Hence, the highly penetrable helium in quiescent regions like Tengchong could migrate through overlying rocks without the need of gas channeling along faults. Similarly, our model can also explain the relationships between Ar concentrations and the distance from springs to faults and magma chambers. In contrast, the geochemical characteristics of other gas components (e.g., CO₂, N₂) are closely associated with the location of major faults. This study offers valuable insights into the spatial relationships between gas geochemistry and subsurface magmatism and fault distribution, highlighting the influence of hydrothermal processes on diverse patterns of gas migration and source in hydrothermal fields.